Elastomers containing surface metalated siliceous filler

ABSTRACT

This invention is based upon the concept of modifying the surface of silica with a metal, such as titanium or zirconium, which will catalyze silanol condensation reactions on the surface of the silica. The utilization of such metalated silica as a filler in rubber compositions results in improved polymer filler interaction and in turn improved physical properties. For instance, such surface metalated siliceous fillers can be used in tire tread compounds to attain improved rolling resistance and treadwear without compromising traction characteristics. The present invention more specifically discloses a tire having a tread comprising (1) a rubbery polymer, (2) a silica coupling agent, and (3) a surface metalated siliceous filler.

This is a divisional of U.S. patent application Ser. No. 12/098,506,filed on Apr. 7, 2008 (now issued as U.S. Pat. No. 7,943,693) whichclaims the benefit of U.S. Provisional Patent Application Ser. No.60/932,477, filed on May 31, 2007. The teachings of U.S. ProvisionalPatent Application Ser. No. 60/932,477 and U.S. patent application Ser.No. 12/098,506 are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to the preparation of silica reinforcedrubber compositions and to articles of manufacture which contain atleast one component comprised thereof, such as tires.

BACKGROUND OF THE INVENTION

Sulfur cured rubber that contains one or more reinforcing fillers isutilized in manufacturing a wide variety of products that require highstrength and abrasion resistance, such as tires. For instance, carbonblack and synthetic amorphous silica, such as precipitated silica, arecommonly used as reinforcing fillers in tires, power transmission belts,conveyor belts, hoses, and a wide variety of other articles ofmanufacture.

Silica is normally used as a filler for rubbery polymers in conjunctionwith a silica coupling agent in order to aid in coupling the silica tothe elastomer, such as natural or synthetic rubber. Such silica couplingagents typically contain a moiety which is reactive with the hydroxylgroups present on the surface of the silica, such as silanol groups, andanother moiety which is interactive with the elastomer being reinforced.

The moiety of the silica coupling agent that reacts with the hydroxylgroups on the surface of the silica is normally a silane-based moietysuch as an alkoxysilane group. Bis (3-ethoxysilylpropyl)polysulfide is arepresentative example of a typical silica coupling agent that containsfrom 2 to about 6 connecting sulfur atoms in its polysulfidic bridge(average of about 2 to about 2.6 connecting sulfur atoms in itspolysulfidic bridge). The interaction of such silica coupling agentswith hydroxyl groups on the surface of the silica generates an alcohol,such as ethanol, and water as reaction byproducts.

U.S. Pat. No. 6,417,286 discloses a method of processing a rubbercomposition which comprises mixing (i) 100 parts by weight of at leastone sulfur vulcanizable elastomer selected from conjugated dienehomopolymers and copolymers and from copolymers of at least oneconjugated diene and aromatic vinyl compound; and (ii) 0.05 to 10 phr ofan titanium or zirconium compound of the formula(R¹—O)_(y)—X—(O—R²—W)_(z) wherein each R¹ is independently selected fromthe group consisting of alkyl radicals having from 1 to 8 carbon atoms;R² is a divalent radical selected from the group consisting of alkyleneshaving 1 to 15 carbon atoms, arylene and alkyl substituted arylenegroups having 6 to 10 carbon atoms; W is an epoxy group; and y is aninteger of from 1 to 3, z is an integer of from 1 to 3 and the sum of yand z equals 4; and X is titanium or zirconium.

U.S. Pat. No. 6,809,135 discloses a tire having a component of a rubbercomposition which comprises (A) elastomer(s) consisting of 100 parts byweight of at least one diene-based elastomer selected from homopolymersand copolymers of isoprene and/or 1,3-butadiene and copolymers of atleast one of isoprene and 1,3-butadiene with a vinyl aromatic compoundselected from styrene and alpha methylstyrene, (B) about 10 to about 150phr of at least one particulate reinforcing filler comprised of about 10to about 100 phr of at least one particulate synthetic silica-basedmaterial having hydroxyl groups on the surface thereof selected from atleast one of aggregates of synthetic amorphous silica, fumed silica, andsilica modified carbon black, and correspondingly, from zero to about 80phr of rubber reinforcing carbon black, and (C) at least oneorgano-metal compound as an organo-tin compound selected from the groupconsisting of dibutyltin dilaurate, di-n-butylbis(2-ethylhexanoate)tin,di-n-butylbis(2,4-pentanedionate)tin, di-n-butyldiacetoxytin,di-n-butyldiacrylatetin, di-n-butyldimethacrylatetin,dimethyldineodecanoatetin, dioctyldilauryltin anddioctyldineodecanoatetin.

United States Patent Application Publication No. 2006/0169391 A1discloses a method for preparing a tire comprising the steps of: mixingingredients including silica and at least one elastomer to form a firstmixture, where the elastomer optionally includes silica-reactivefunctionalized elastomer; cooling the first mixture; further mixing thefirst mixture, optionally with additional ingredients including a silicacoupling agent, a silica reactive dispersing agent or both, to form anintermediate mixture, with the proviso that at least one of theingredients mixed to form the first mixture or the additionalingredients added to form the intermediate composition includes asilica-reactive compound; adding ingredients including a curative to theintermediate mixture to form a vulcanizable mixture; mixing thevulcanizable mixture; forming the vulcanizable mixture into a tirecomponent; building a tire by including the tire component; curing thetire; where a titanium compound is added to at least one of said step ofmixing ingredients to form a first mixture or said step of furthermixing to form an intermediate mixture.

The importance of attaining good to compatibility between rubbers andfillers used in tires and other industrial products is well appreciated.For example, in tire tread compounds improved compatibility between therubber and the filler, such as carbon black or silica, normally resultsin lower hysteresis and improved tire tread life. Low hysteresis in tiretread compounds is desirable because it is indicative of lower heatgeneration during rolling and lower rolling resistance which leads tobetter fuel economy. U.S. Pat. No. 7,108,033 discloses rubbercompositions that attain improved compatibility with the types offillers that are typically used in rubber compounds, such as carbonblack and silica, through the utilization of rubbery polymers thatcontain repeat units that are derived from one or more conjugateddiolefin monomers and at least one monomer that is functionalized with aleaving group, such as a halogen.

U.S. Pat. Nos. 6,693,160, 6,753,447 6,812,307, 6,825,306, 6,901,982,6,933,358, 6,936,669, 7,041,761, 6,627,721, 6,627,722, 6,630,552,6,664,328, 6,790,921, and 6,927,269 disclose the incorporation offunctionalized monomers into rubbery polymers to improve compatibilitywith fillers.

SUMMARY OF THE INVENTION

This invention is based upon the concept of modifying the surface ofsilica with a metal, such as titanium or zirconium, which will catalyzesilanol condensation reactions on the surface of the silica. In otherwords, modifying the surface of silica with titanium or zirconiumprovides a catalyst for the silanol condensation reaction between thesilica surface and silane coupling agents used in silica filledcompounds. The utilization of such metalated silica as a filler inrubber compositions results in improved polymer filler interaction andin turn improved physical properties. For instance, such surfacemetalated siliceous fillers can be used in tire tread compounds toattain improved rolling resistance and treadwear without compromisingtraction characteristics.

The present invention more specifically discloses an elastomericcomposition which is comprised of (1) a rubbery polymer which iscomprised of repeat units which are derived from at least one conjugateddiolefin monomer, (2) a silica coupling agent, and (3) a surfacemetalated siliceous filler. In these elastomeric compositions thesurface metalated siliceous filler is typically metalated with acatalytically active metal selected from the group consisting oftitanium, zirconium, niobium, tantalum, hafnium, nickel, copper, tin,zinc, cobalt, antimony, manganese, chromium, vanadium, molybdenum, andiron. The catalytically active metal can also be a lanthanide metal,such as lanthanum, cerium, praseodymium, neodymium, promethium,samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, ytterbium, or lutetium.

The subject invention further reveals a tire which is comprised of agenerally toroidal-shaped carcass with an outer circumferential tread,two spaced beads, at least one ply extending from bead to bead andsidewalls extending radially from and connecting said tread to saidbeads, wherein said tread is adapted to be ground-contacting, andwherein said tread is comprised of (1) a rubbery polymer which iscomprised of repeat units which are derived from at least one conjugateddiolefin monomer, (2) a silica coupling agent, and (3) a surfacemetalated siliceous filler.

The present invention also discloses a method of producing anelastomeric composition that is reinforced with a surface metalatedsiliceous filler, said method comprising the steps of: (1) chemisorptinga metal compound onto the surface of silica to produce a surfacemetalated siliceous filler, and (2) blending the surface metalatedsiliceous filler and a silica coupling agent into a rubbery polymer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an idealized concept of efficient coupling withsilica that is attained by practicing this invention as compared to theprior art.

DETAILED DESCRIPTION OF THE INVENTION

The surface metalated siliceous fillers of this invention can be used toreinforce virtually any rubbery polymer, such as rubbers that arederived from conjugated diolefin monomers. For purposes of thisinvention natural rubber is considered to be derived from a conjugateddiolefin monomer since it contains isoprene repeat units. The surfacemetalated siliceous fillers of this invention are of particular value inthe reinforcement of tire polymers, such as rubber blends utilized intire tread rubber compositions. In any case, the surface metalatedsiliceous fillers of this invention can be used to reinforce naturalrubber, synthetic polyisoprene rubber, high cis-1,4-polybutadienerubber, medium vinyl polybutadiene rubber, solution styrene-butadienerubber (SBR made by solution polymerization), emulsion styrene-butadienerubber (SBR made by emulsion polymerization), styrene-isoprene rubber,styrene-isoprene-butadiene rubber (SIBR), isoprene-butadiene rubber(IBR), and various blends of such rubbery polymers. The rubbery polymercan be coupled or functionalized by polymerizing a functionalizedmonomer therein or by synthesizing the rubbery polymer utilizing afunctionalized initiator. The teachings of U.S. Pat. Nos. 6,693,160,6,753,447, 6,812,307, 6,825,306, 6,901,982, 6,933,358, 6,936,669,7,041,761, 6,627,721, 6,627,722, 6,630,552, 6,664,328, 6,790,921, and6,927,269 are incorporated by reference herein for the purpose ofillustrating rubbery polymers having functionalized monomersincorporated therein that can be utilized in the practice of thisinvention.

The rubbery polymers that are reinforced with the surface metalatedsiliceous fillers of this invention (elastomeric compositions) alsocontain at least one silica coupling agent. The silica coupling agentwill typically be a mercaptosilane, a blocked mercaptosilane, or anorganosilicon compound of the general formula:Z-Alk-S_(n)-Alk-Z  (I)in which Z is selected from the group consisting of:

wherein R¹ is an alkyl group containing from 1 to 4 carbon atoms, acyclohexyl group, or a phenyl group; wherein R² is an alkoxy groupcontaining from 1 to 8 carbon atoms, or a cycloalkoxy group containingfrom 5 to 8 carbon atoms; wherein Alk is a divalent hydrocarbon of 1 to18 carbon atoms and wherein n represents an integer from 2 to 8. Themercaptosilanes and blocked mercaptosilanes that can be used in thepractice of this invention are described in International PatentPublication No. WO 2006/076670. The teachings of WO 2006/076670 areincorporated herein by reference for the purpose of describing specificmercaptosilanes and blocked mercaptosilanes that can be used in thepractice of this invention. The teachings of WO 03091314 are alsoincorporated herein by reference for the purpose of describing specificsilanes that can be utilized in the practice of this invention whichemit low levels of volatile organic compounds or no volatile organiccompounds.

Specific examples of sulfur containing organosilicon compounds which maybe used as the silica coupling agent in accordance with the presentinvention include: 3,3′-bis(trimethoxysilylpropyl) disulfide,3,3′-bis(triethoxysilylpropyl)tetrasulfide,3,3′-bis(triethoxysilylpropyl)octasulfide,3,3′-bis(trimethoxysilylpropyl)tetrasulfide,2,2′-bis(triethoxysilylethyl)tetrasulfide,3,3′-bis(trimethoxysilylpropyl)trisulfide,3,3′-bis(triethoxysilylpropyl)trisulfide,3,3′-bis(tributoxysilylpropyl)disulfide,3,3′-bis(trimethoxysilylpropyl)hexasulfide,3,3′-bis(trimethoxysilylpropyl)octasulfide,3,3′-bis(trioctoxysilylpropyl)tetrasulfide,3,3′-bis(trihexoxysilylpropyl)disulfide,3,3′-bis(tri-2″-ethylhexoxysilylpropyl)trisulfide,3,3′-bis(triisooctoxysilylpropyl)tetrasulfide,3,3′-bis(tri-t-butoxysilylpropyl)disulfide, 2,2′-bis(methoxy diethoxysilyl ethyl)tetrasulfide, 2,2′-bis(tripropoxysilylethyl)pentasulfide,3,3′-bis(tricyclonexoxysilylpropyl)tetrasulfide,3,3′-bis(tricyclopentoxysilylpropyl)trisulfide,2,2′-bis(tri-2″-methylcyclohexoxysilylethyl)tetrasulfide,bis(trimethoxysilylmethyl)tetrasulfide, 3-methoxy ethoxy propoxysilyl3′-diethoxybutoxy-silylpropyltetrasulfide, 2,2′-bis(dimethylmethoxysilylethyl)disulfide, 2,2′-bis(dimethylsec.butoxysilylethyl)trisulfide, 3,3′-bis(methylbutylethoxysilylpropyl)tetrasulfide, 3,3′-bis(dit-butylmethoxysilylpropyl)tetrasulfide, 2,2′-bis(phenyl methylmethoxysilylethyl)trisulfide, 3,3′-bis(diphenylisopropoxysilylpropyl)tetrasulfide, 3,3′-bis(diphenylcyclohexoxysilylpropyl)disulfide, 3,3′-bis(dimethylethylmercaptosilylpropyl)tetrasulfide, 2,2′-bis(methyldimethoxysilylethyl)trisulfide, 2,2′-bis(methylethoxypropoxysilylethyl)tetrasulfide, 3,3′-bis(diethylmethoxysilylpropyl)tetrasulfide, 3,3′-bis(ethyldi-sec.butoxysilylpropyl)disulfide, 3,3′-bis(propyldiethoxysilylpropyl)disulfide, 3,3′-bis(butyldimethoxysilylpropyl)trisulfide, 3,3′-bis(phenyldimethoxysilylpropyl)tetrasulfide, 3-phenyl ethoxybutoxysilyl3′-trimethoxysilylpropyl tetrasulfide,4,4′-bis(trimethoxysilylbutyl)tetrasulfide,6,6′-bis(triethoxysilylhexyl)tetrasulfide, 12,12′-bis(triisopropoxysilyldodecyl)disulfide, 18,18′-bis(trimethoxysilyloctadecyl)tetrasulfide,18,18′-bis(tripropoxysilyloctadecenyl)tetrasulfide,4,4′-bis(trimethoxysilyl-buten-2-yl)tetrasulfide,4,4′-bis(trimethoxysilylcyclohexylene)tetrasulfide,5,5′-bis(dimethoxymethylsilylpentyl)trisulfide,3,3′-bis(trimethoxysilyl-2-methylpropyl)tetrasulfide,3,3′-bis(dimethoxyphenylsilyl-2-methylpropyl)disulfide.

The preferred sulfur containing organosilicon compounds are the3,3′-bis(trimethoxy or triethoxy silylpropyl)sulfides. The mostpreferred compound is 3,3′-bis(triethoxysilylpropyl)tetrasulfide.Therefore with respect to formula I, Z is preferably

wherein R² is an alkoxy of 2 to 4 carbon atoms, with 2 carbon atomsbeing particularly preferred; Alk is a divalent hydrocarbon of 2 to 4carbon atoms with 3 carbon atoms being particularly preferred; and n isan integer of from 3 to 5 with 4 being particularly preferred.

The amount of the silica coupling agent that should be incorporated intothe elastomeric compositions of this invention will vary depending onthe level of the siliceous fillers that are included in the rubberycomposition. Generally speaking, the amount of the silica coupling agentused will range from about 0.01 to about 5 parts by weight per part byweight of the siliceous fillers. Preferably, the amount of silicacoupling agent utilized will range from about 0.02 to about 1 parts byweight per part by weight of the siliceous fillers. Preferably, theamount of silica coupling agent utilized will range from about 0.04 toabout 0.4 parts by weight per part by weight of the siliceous fillers.More preferably the amount of the silica coupling agent included in theelastomeric compositions of this invention will range from about 0.05 toabout 0.25 parts by weight per part by weight of the siliceous fillers.

The rubbery compositions of this invention can be compounded utilizingconventional ingredients and standard techniques. For instance, theelastomeric compositions of this invention will typically be mixed withcarbon black, sulfur, additional fillers, accelerators, oils, waxes,scorch inhibiting agents, and processing aids in addition to the surfacemetalated siliceous filler. It should also be noted that conventionalsilica fillers can also be incorporated into such compositions. Forinstance, the standard siliceous pigments that are commonly used inrubber compounding applications can be used in addition to the surfacemetalated siliceous filler. For instance the silica can includepyrogenic and precipitated siliceous pigments (silica), althoughprecipitate silicas are preferred. The siliceous pigments preferablyemployed in this invention are precipitated silicas such as, forexample, those obtained by the acidification of a soluble silicate, suchas sodium silicate. Such silicas can typically be characterized byhaving a BET surface area, as measured using nitrogen gas, which iswithin the range of about 40 to about 600 square meters per gram (m²/g),and more preferably in a range of about 50 to about 300 m²/g. The BETmethod of measuring surface area is described in the Journal of theAmerican Chemical Society, Volume 60, page 304 (1930).

The silica that can be used in the elastomeric compositions of thisinvention can also typically be characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300. The silica typically hasan average ultimate particle size which is within the range of 0.01 to0.05 micron as determined using an electron microscope, althoughspecific silica particles may be even smaller, and sometimes larger insize.

Various commercially available silicas may be used in the practice ofthis invention. Some representative examples of such silicas includethose from PPG Industries that are sold under the Hi-Sil trademark withdesignations 210 and 243, silicas available from Rhone-Poulenc with thedesignations of Z1165MP and Z165GR, and silicas available from DegussaAG with the designations VN2 and VN3.

In most cases, the elastomeric compositions of this invention will becompounded with sulfur and/or a sulfur containing compound, at least onefiller, at least one accelerator, at least one antidegradant, at leastone processing oil, zinc oxide, optionally a tackifier resin, optionallya reinforcing resin, optionally one or more fatty acids, optionally apeptizer and optionally one or more scorch inhibiting agents. Suchblends will normally contain from about 0.5 to 5 phr (parts per hundredparts of rubber by weight) of sulfur and/or a sulfur containing compoundwith 1 phr to 2.5 phr being preferred. It may be desirable to utilizeinsoluble sulfur in cases where bloom is a problem.

Normally the total amount of fillers utilized in the elastomeric blendsof this invention will be within the range of 10 phr to 150 phr with itbeing preferred for such blends to contain from 30 phr to 80 phrfillers. As has been explained, the filler can be comprised solely ofthe surface metalated siliceous filler. However, in most cases at leastsome carbon black will be utilized in such elastomeric compositions. Ifcarbon black is also present, the amount of carbon black, if used, mayvary. Generally speaking, the amount of carbon black will vary fromabout 5 phr to about 80 phr. Preferably, the amount of carbon black willrange from about 10 phr to about 40 phr. Clays and/or talc can beincluded in the filler to reduce cost. Starch can also be included toattain good results in some cases. In any case, the blend will alsonormally include from 0.1 to 2.5 phr of at least one accelerator with0.2 phr to 1.5 phr being preferred. Antidegradants, such as antioxidantsand antiozonants, will generally be included in the rubbery blends ofthis invention in amounts ranging from 0.25 phr to 10 phr with amountsin the range of 1 phr to 5 phr being preferred. Processing oils willgenerally be included in the blend in amounts ranging from 2 phr to 100phr with amounts ranging from 5 phr to 50 phr being preferred. Theelastomeric blends of this invention will also normally contain from 0.5phr to 10 phr of zinc oxide with 1 to 5 phr being preferred. Theseblends can optionally contain up to about 10 phr of tackifier resins, upto about 10 phr of reinforcing resins, 1 phr to 10 phr of fatty acids,up to about 2.5 phr of peptizers, and up to about 1 phr of scorchinhibiting agents.

The elastomeric compositions of this invention which include a surfacemetalated siliceous filler and a silica coupling agent will typically bemixed utilizing a thermomechanical mixing technique. The mixing of thetire tread rubber formulation can be accomplished by methods known tothose having skill in the rubber mixing art. For example the ingredientsare typically mixed in at least two stages, namely at least onenon-productive stage followed by a productive mix stage. The finalcuratives including sulfur vulcanizing agents are typically mixed in thefinal stage which is conventionally called the “productive” mix stage inwhich the mixing typically occurs at a temperature, or ultimatetemperature, lower than the mix temperature(s) than the precedingnon-productive mix stage(s). The rubber, the surface metalated siliceousfiller and the silica coupling agent are mixed in one or morenon-productive mix stages. The terms “non-productive” and “productive”mix stages are well known to those having skill in the rubber mixingart. The non-productive mixing stage includes the mixing of componentsof the blend before curatives are added. The productive mixing stage isthe mixing step in which sulfur and/or other curatives are added to theblend.

The sulfur vulcanizable rubber composition containing the sulfurcontaining silica coupling agent, vulcanizable rubber and generally atleast part of the silica should be subjected to a thermomechanicalmixing step. The thermomechanical mixing step generally comprises amechanical working in a mixer or extruder for a period of time suitablein order to produce a rubber temperature between 140° C. and 190° C. Theappropriate duration of the thermomechanical working varies as afunction of the operating conditions and the volume and nature of thecomponents. For example, the thermomechanical working may be for aduration of time which is within the range of about 2 minutes to about20 minutes. It will normally be preferred for the rubber to reach atemperature which is within the range of about 145° C. to about 180° C.and to be maintained at said temperature for a period of time which iswithin the range of about 1 minutes to about 12 minutes. It willnormally be more preferred for the rubber to reach a temperature whichis within the range of about 155° C. to about 170° C. and to bemaintained at said temperature for a period of time which is within therange of about 2 minutes to about 6 minutes.

Tire tread compounds made using such rubber blends that contain surfacemetalated siliceous fillers can be incorporated into tire treads usingstandard tire manufacturing techniques. For instance, tires can be builtutilizing such conventional procedures with the rubber blend containingthe surface metalated siliceous filler simply being substituted for therubber compounds typically used as the tread rubber. In other words, thesurface metalated siliceous filler can be employed to replace all or aportion of the conventional silica filler that can be utilized in makingtire tread compounds. For instance, such a rubber blend for anautomobile tire can be a mixture of solution styrene-butadiene rubberwith cis-1,4-polybutadiene rubber. Such tire tread formulations willtypically contain at least 40 phr (parts by weight per 100 parts ofrubber by weight) of the solution styrene-butadiene rubber and at least20 phr of the cis-1,4-polybutadiene rubber. Other useful tire treadrubber formulations for automobile tires can be blends of emulsionstyrene-butadiene rubber with cis-1,4-polybutadiene rubber and/ornatural rubber. Useful tire tread formulations for truck tires can beblends of natural rubber with cis-1,4-polybutadiene rubber. Such blendswill normally contain at least 45 phr of the natural rubber and at least20 phr of the cis-1,4-polybutadiene rubber.

After the tire has been built with the elastomeric blend containing thesurface metalated siliceous filler it can be vulcanized using a normaltire cure cycle. Tires made in accordance with this invention can becured over a wide temperature range. However, it is generally preferredfor the tires to be cured at a temperature ranging from about 132° C.(270° F.) to about 166° C. (330° F.). It is more typical for the tiresof this invention to be cured at a temperature ranging from about 143°C. (290° F.) to about 154° C. (310° F.). It is generally preferred forthe cure cycle used in the vulcanization of tires to be of a durationwhich is within the range of about 10 to about 20 minutes with a curecycle of about 12 to about 18 minutes being most preferred.

The present invention involves modifying the surface of silica withtetravalent titanium through a reaction with silanols on the silicasurface. Such a treatment procedure can be depicted as follows:

The resulting surface metalated siliceous filler then has a built incatalyst for a silanol condensation reaction between the silica and thesilica coupling agent. The titanium catalyzation of a silanolcondensation reaction can be depicted as follows:

It should be noted that most of the common silica coupling agents usedin rubber compounding can either condense on the silica surface or selfcondense. Since these two reactions compete with each other having acatalyst at the silica surface will result in more efficient couplingbetween the silica filler and the elastomer by favoring either thecoupler to condense on the silica surface or to self condense around thesilica surface, or a combination of both reactions.

FIG. 1 depicts the coupling that is attained by practicing thisinvention as compared to conventional coupling. As can be seen, thepolymer chains 1 have more interaction with the silica 2 when thecoupling agent 3 is around the surface of the silica (as in thisinvention) as compared to when the silica coupling agent 3 is dispersedthroughout the elastomer matrix as is the case with the prior art. Inany case, FIG. 1 illustrates an idealized concept of efficient couplingwith silica that is attained by practicing this invention.

Surface metalated siliceous fillers can be made using a variety oftechniques. For example, titanium modified silica can be easily made byat least three different methods. One is a solution method where thesilica is placed in cyclohexanol, water is removed via azeotrope andthen the titanium as tetraisopropyl titanate (Tyzor® TPT from DuPont) isadded to the solution with heat. The titanium modified silica from thissolution method can be added to rubber compounds to attain an increasedmodulus and a decreased level of hysteresis. A second method wasdeveloped for ease of preparation. In this method silica is dried. Thiscan be done at 225° C. in a vacuum oven equipped with a vacuum pump andcold trap. Then, a solution of Tyzor® tetraisopropyl titanate inisopropanol can be sprayed onto the silica surface. The resulting silicacan then be dried. This can again be done in a vacuum oven at 225° C.This titanium treated silica can then be added to rubber compounds toattain lower hysteresis at equivalent levels of stiffness. It isimportant to dry the silica before application of the tetraisopropyltitanate. No significant improvement is attained in cases where thetetraisopropyl titanate is sprayed onto the surface of the silicawithout first drying it.

Conventional sol-gel techniques for commercially manufacturing amorphousrubber reinforcing silica can be modified to make surface metalatedsiliceous fillers. This third method for making surface metalatedsiliceous fillers involves adding a reactive metal, such as a reactivetitanium compound, to an aqueous suspension of silica prior to itsisolation and drying in the conventional sol-gel technique.

Titanium in the form of titanium chelates is also available from Dupont.These chelated titaniums are more resistant to hydrolysis thantetraisopropyl titanate (TPT). Three of these chelates were tried in aneffort to eliminate the step of drying the silica. The triethanolaminechelated titanium (Tyzor® TE) showed similar compound propertyenhancements when sprayed onto silica without drying as did the Tyzor®TPT treatment of dried silica. Further compound studies showed that thelow hysteresis values in the compounds can be duplicated with increasedloadings of amine accelerators, however using these amines results in asignificant drop in compound tear properties when compared to thetitanium treated silica compounds.

The results obtained to date with the titanium modified silica indicatethat lower hysteresis with equivalent or higher modulus can be obtained.The higher tear values at similar hysteresis indicate that this conceptcan shift the trade-off between hysteresis, stiffness, and tear. Thisshould result in a tire with lower rolling resistance and equivalent orbetter tread wear characteristics without compromising tractioncharacteristics.

The term “siliceous filler” is used herein to describe siliceousreinforcing fillers that contain domains of silica which are accessiblefor potential surface reactions with the ingredients commonly used toproduce silica-reinforced elastomers. A wide variety of siliceousfillers are suitable for utilization in the practice of this invention.In one embodiment of this invention, the siliceous filler is hydratedsilica, which can be represented by the formula (SiO₂)(H₂O)_(n), whereinn represents a number of less than one. Thus, n represents the totalamount of water that, in principle, can be obtained by completelydehydrating the hydrated silica to silicon dioxide (SiO₂). It should beunderstood that the water may be present in hydrated silica as eitherhydrogen-bonded water or in populations of SiOH groups that can react atprogressively higher temperatures to produce H₂O and siloxane linkages(Si—O—Si) in silica. The value of n in the formula for hydrated silicadepends greatly upon the method used to manufacture, dry and store thehydrated silica. In any case, the siliceous filler used in the practiceof this invention can be virtually any reinforcing hydrated silica. Thesurface of the siliceous filler will typically be essentially void ofmetals that catalyze the reaction of silane couplers with hydratedsiliceous fillers, except in small amounts as impurities.

In another embodiment of this invention, the siliceous filler can be anetwork of hydrated silica with partial replacement of some silica(SiO₂) with one or more oxides of another element. In this embodiment,the siliceous filler can be represented by the formula(SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n), wherein M represents one or more metalscapable of forming a network structure with silicon and oxygen, whereinn represents a number of less than one representing the total amount ofwater that, in principle, can be obtained by completely dehydrating thesiliceous filler to form (SiO₂)(M_(x)O_(y))_(m), and wherein x and y areintegers corresponding to molar ratios in oxide compounds of M.

In a further embodiment of this invention, the siliceous filler can berepresented by the formula (SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n), wherein xrepresents 2, wherein y represents 3, and M represents an element knownto produce stable trivalent ions in aerobic, aqueous solutions, such asaluminum. In another embodiment of this invention, the siliceous fillercan be represented by the formula (SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n),wherein x represents 1, wherein y represents 2, and wherein M is anelement known to produce stable tetravalent ions in aerobic, aqueoussolutions, such as titanium, zirconium, germanium or tin. Titanium,zirconium, and tin are preferred with titanium and zirconium being morepreferred. Normally, titanium is the very most preferred.

The siliceous filler can also be represented by the formula(SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n), wherein x represents 1, wherein yrepresents 1, and wherein M is an element known to produce stabledivalent ions in aerobic, aqueous solutions, such as magnesium, calcium,zinc, or tin. The siliceous filler can also be represented by theformula (SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n), wherein x represents 2, whereiny represents 1, and wherein M is an element known to produce stablemonovalent ions in aerobic, aqueous solutions, such as lithium, sodium,or potassium. The siliceous filler can contain (M_(x)O_(y)) groups thathave various combinations of metals. For instance, the family ofhydrated siliceous filler containing a 1:1 ratio of Al to Mg would berepresented by the formula (SiO₂)(MgAlO_(2.5))_(m)(H₂O)_(n), wherein mis the molar ratio of (MgAlO_(2.5)) to (SiO₂) and n is a number lessthan one that represents the total amount of water that, in principle,can be obtained by completely dehydrating the siliceous filler to(SiO₂)(MgAlO_(2.5))_(m).

In siliceous fillers of the formula: (SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n),(SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n), and (SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n)other metals, such as aluminum and/or boron can be present on thesurface of the siliceous filler provided that the other metal does notact as the predominant catalytically relevant species. However, thesurface of the siliceous filler can be void or essentially free of suchadditional metals in one embodiment of this invention.

In cases where hydrated silica is used as the siliceous filler, itshould be understood that: (1) n in the formula(SiO₂)(M_(x)O_(y))_(m)(H₂O)_(n) represents the total amount of waterthat can be obtained by completely dehydrating the material to(SiO₂)(M_(x)O_(y))_(m), (2) the water represented in the formula may bepresent as either hydrogen-bonded H₂O or populations of SiOH or MOHgroups that can react at progressively higher temperatures to produceH₂O and linkages between Si and M, such as Si—O—Si, Si—O-M, or M-O-M,and (3) the value of n is not a limiting factor with respect to thesiliceous fillers that can be used in accordance with this invention.

The terms “chemisorption” and “chemisorbed” are used herein to describethe chemical reaction of metal compounds with siliceous fillers and theresults from the reactions of metal compounds with siliceous fillers.Chemisorption is any process by which a metal compound reacts withsiliceous filler to produce a chemically-modified siliceous filler viaformation of one or more chemical bonds between the siliceous filler andthe groups comprising the metal compound. When attached to siliceousfiller via chemical bonds, the metal compound is said to be“chemisorbed” and the resulting chemically-modified siliceous product isreferred to as being surface metalated siliceous filler.

The surface chemistry of silica is dominated by the reaction chemistryof surface silanol groups (SiOH) and strained siloxane (Si—O—Si)linkages. Depending on the specific method used to manufacture thesilica, such as the process by which it is dried and the conditions forits storage (humidity level), it is possible to produce silica that canreact with metal compounds to produce a broad range of chemisorbedspecies. The same is true for the siliceous fillers described herein. Insome cases, metal ions from the metal compound can become attached tooxygen atoms from silanol groups on the siliceous filler. In othercases, surface silanol groups from the siliceous filler can formchemical bonds to the metal compound via hydrogen-bonding to atomspossessing Bronsted-base lone pairs of electrons. These and otherchemisorption reactions are well known to those skilled in the art, andthey are essential for producing the surface metalated siliceous fillersneeded in the practice of this invention.

The siliceous filler can be metalated by (1) chemisorption with chemicalbonds between the siliceous filler and the metal compound formingwithout breaking chemical bonds in the original metal compound, (2)chemisorption that involves breakage of chemical bonds in both thesiliceous filler and the metal compound, or (3) a combination of both.Chemisorption can occur without breaking chemical bonds of the originalmetal compound by (i) simple coordination of Lewis-acidic metalcompounds to oxygen atoms from surface SiOH groups or MOH groups (Mrepresents a metal capable of forming a network structure with siliconand oxygen as depicted in the formula: represents a metal selected fromthe group consisting of titanium, zirconium, tin, and iron), (ii)hydrogen-bonding of acidic metal compounds to surface SiOH groups or MOHgroups, or (iii) hydrogen bonding of surface SiOH groups or MOH groupsto the metal compound via atoms possessing Bronsted-base lone pairs ofelectrons. Chemisorption can occur by breakage of chemical bonds in boththe siliceous and the metal compound by (i) reaction of surface SiOHgroups with metal compounds containing M-R groups, wherein R is anorganyl group (including hydrogen) susceptible to protonolysis by SiOH,(ii) reaction of surface SiOH groups with metal compounds containing M-Xgroups, wherein X represents a monovalent group susceptible tometathesis reactions with SiOH, such as alkoxide groups (RO), amidogroups (R¹R²N), thiolate groups (RS), halides (F, Cl, Br, or I), andpseudohalide groups (N₃, SCN), (iii) displacement of neutral ligandsfrom the metal compound by oxygen atoms from surface SiOH groups, and(iv) addition of H—X bonds, wherein X is a heteroatom from a group onthe metal compound, to strained surface Si—O—Si linkages to produce newSi—OH and Si—X bonds.

This invention is illustrated by the following examples that are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, parts and percentages are givenby weight.

Surface metalated siliceous fillers can be made by a solution techniqueor by a spray technique. These techniques can be depicted as follows:

Solution Method

Zeosil 1165 hydrated amorphous silica from Rhodia was placed into a highspeed mixer for 30 seconds in order to break apart the micropearlagglomerates. Approximately 400 ml of cyclohexanol was added to a 2liter resin kettle equipped with a Dean-Stark trap and an additionfunnel. The cyclohexanol was stirred at reflux with bubbling nitrogenfor 1 hour. The silica was then added to the solvent and the solutionwas allowed to reflux until no more water was collected in theDean-Stark trap. A total of 13 ml of water was collected. At this point10.5 ml of Tyzor®TPT (tetraisopropyl titanate) was added dropwise withstirring to the solution. The solution was stirred for 2 hours and thencooled. The solution was transferred to a round bottom flask and thecyclohexanol removed with a rotovap. The remaining solid was then driedin a vacuum oven at 220° C. overnight.

Spray Method

Zeosil 1165 hydrated amorphous silica from Rhodia was broken up asabove. The silica was then placed in a vacuum oven at 220° C. and driedfor 3 hours. A 50/50 v/v solution of Tyzor® TPT and isopropanol wasplaced in an atomizing spray gun. The silica removed from the vacuumoven and was sprayed with this solution while still hot. The silica wasthen placed back into the vacuum oven overnight.

Example I Preparation of Silica Composites

Silica composites of chemically modified precipitated silica having aplurality of transition metal ions in a form of a titanium compoundchemisorbed on the surface of the precipitated silica are prepared andreferred to in this example as Silica Composites 1 through 5.

The silica composites were prepared by three different methods. In thesolution method, precipitated silica was placed into a high speed mixerfor 30 seconds in order to break apart the micropearl agglomerates.Approximately 400 ml of cyclohexanol was added to a 2 liter resin kettleequipped with a Dean-Stark trap and an addition funnel. The cyclohexanolwas stirred at reflux with bubbling nitrogen for 1 hour. The silica wasthen added to the solvent and the solution was allowed to reflux untilno more water was collected in the Dean-Stark trap. A total of 13 ml ofwater was collected. At this point 10.5 ml of Tyzor® TPT (tetraisopropyltitanate) was added dropwise with stirring to the solution. The solutionwas stirred for 2 hours and then cooled. The solution was transferred toa round bottom flask and the cyclohexanol removed with a rotovap. Theremaining solid was then dried in a vacuum oven at 220° C. overnight.

A second method used to prepare the silica composites was a spraymethod. In the spray method, precipitated silica was broken up as above.The silica was then placed in a vacuum oven at 220° C. and dried for 3hours under vacuum. A 50/50 v/v solution of Tyzor® TPT and isopropanolwas placed in a atomizing spray gun. The silica removed from the vacuumoven and was sprayed with this solution while still hot. The silica wasthen placed back into the vacuum oven overnight.

A third method was used in which the silica was prepared the same as thesecond method without the drying steps.

TABLE 1 Silica Composite Sample 1 2 3 4 Solution Method X (1) SprayMethod (2) X Spray Method (no X X drying) (3) Titanium Reagent TyzorTPT¹ Tyzor TPT¹ Tyzor TPT¹ Tyzor TE² Used ¹Tyzol TPT is tetraisopropyltitanate. ²Tyzor TE is (triethanolamine)isopropyl titanate.

Example II Preparation of Rubber Samples

Rubber samples containing silica composites of Example I are preparedand referred to in this example as Rubber Samples A through E, withRubber Sample A being a Control Rubber Sample.

The Rubber Samples containing silica composites were prepared by twostage mixing in a 1,500 cc Banbury mixer.

For the preparation of the Rubber Samples, in the first, ornon-productive mixing stage, the ingredients are mixed for about 3minutes to an autogeneously generated, via the high shear mixing in theinternal rubber mixer, drop temperature of about 150° C. at which timethe batch is “dropped”, or removed, from the associated internal rubbermixer. The batch is sheeted out and allowed to cool to a temperaturebelow 40° C., namely below 30° C. The batch is then mixed in aproductive mixing stage during which free sulfur and vulcanizationaccelerator(s) are added for a period of about 2 minutes to a droptemperature of about 110° C.

The rubber formulations are illustrated in Table 2.

TABLE 2 Control Sample A Sample B Sample C Sample D Sample E Rubber ¹ 7070 70 70 70 Rubber ² 30 30 30 30 30 Silica ³ 50 50 Silica Composite ⁴ 50Silica Composite ⁵ 50 Silica Composite ⁶ 50 Coupling Agent ⁷ 8 8 8 8 8Fatty Acid ⁸ 1 1 1 1 1 Zinc oxide 2.5 2.5 2.5 2.5 2.5 Tetraisopropyl 2.5titanate PRODUCTIVE STAGE sulfur 1 1 1 1 1 Accelerator(s) ⁹ 1.3 1.3 1.31.3 1.3 Antidegradants ¹⁰ 1 1 1 1 1 ¹ Obtained as S-SBR from TheGoodyear Tire & Rubber Company as Solflex  ®1810 ² Naturalcis-polyisoprene as SMR-20 Natural Rubber ³ Obtained as precipitated,hydrated amorphous silica from Rhodia as Zeosil 1165 ⁴ Silica CompositeNo. 1 from Example I ⁵ Silica Composite No. 2 from Example I ⁶ SilicaComposite No. 3 from Example I ⁷ As3.3′-bis(triethoxysilylpropyl)tetrasulfide from the Degussa Corporationas Si69 ⁸ Primarily stearic acid ⁹ Sulfenamide and guanidine basedsulfur cure accelerators ¹⁰ Antidegradants as mixed PPD's

The cure behavior and various cured physical properties of therespective samples are shown in the Table 3. The samples wereindividually cured for about 30 minutes at a temperature of about 150°C.

TABLE 3 Control Test Properties Sample A Sample B Sample C Sample DSample E Rheometer, 150° C. Maximum Torque 30.46 22.08 24.87 26.76 24.25(dNm) Minimum Torque 5.66 5.34 5.34 5.91 5.43 (dNm) T90 29.9 25.7 23.4424.1 40.4 Stress Strain (cured 40 minutes at 150° C. Tensile Strength(MPa) 21.24 21.97 20.26 17.95 17.95 Elongation at break (%) 499 472 489428 549 300% Modulus (MPa) 8.9 9.6 10.5 10.7 7.5 RPA (150° C. curecycle, 11 Hz, 100° C.) G′, 10% strain 1594 1812 2102 1775 1937 Tan δ,10% strain 0.15 0.11 0.11 0.14 0.16

From Table 3 it can be seen that the tan δ values of Samples B and C aresignificantly lower than those of Samples A, D, and E.

This is considered herein to be significant in that tan δ is ameasurement of the hysteresis of the rubber compound being measured. Thehysteresis is indicative of the rolling resistance of a rubber compoundwhen used in a tire.

From Table 3 it can further be seen that the 300% modulus and tensilestrength of Samples B and C are higher than those for Sample E.

This is considered herein to be significant in that improved tensileproperties along with the lower hysteresis is indicative of improvedpolymer filler interaction. This shows that pretreating the silica bychemisorptions of the titanium compounds results in improved propertiesversus adding the titanium compounds directly to the mixer.

Example III Preparation of Rubber Samples

Rubber samples containing silica composites of Example I are preparedand referred to in this Example as Rubber Samples F through H, withRubber Sample F being a Control Rubber Sample. The rubber samplescontaining the silica composites were mixed as in Example II. The rubberformulations are illustrated in Table 4.

TABLE 4 Control Sample F Sample G Sample H Rubber ¹ 25 25 25 Rubber ² 2626 26 Rubber ³ 49 49 49 Silica ⁴ 73 73 Silica Composite ⁵ 73Triethanolamine, 5.6 isopropyl-titanate Coupling Agent ⁶ 6.5 6.5 6.5Fatty Acid ⁷ 3 3 3 Zinc oxide 2 2 2 Rubber processing oil ⁸ 30 30 30 Wax⁹ 1.5 1.5 1.5 Resin ¹⁰ 3.5 3.5 3.5 Antidegradants 2.75 2.75 2.75PRODUCTIVE STAGE sulfur 2.18 2.18 2.18 Accelerator(s)11 1.3 1.3 1.3 ¹Tin coupled amino functionalized solution copolymer of butadiene andstyrene, SSBR, with 21% bound styrene. ² Obtained as Budene ® 1207cis-polybutadiene rubber from The Goodyear Tire & Rubber Company. ³ Asolution polymerized copolymer of butadiene and styrene, SSBR, with 33%bound styrene. ⁴ Obtained as precipitated, hydrated amorphous silicafrom Rhodia as Zeosil 1165. ⁵ Silica Composite No. 4 from Example I.

The cure behavior and various cured physical properties of therespective samples are shown in the subsequent Table 5. The samples wereindividually cured for about 30 minutes at a temperature of about 150°C.

TABLE 5 Control Test Properties Sample F Sample G Sample H Rheometer,150° C. Maximum Torque 19.58 17.46 18.32 (dNm) Minimum Torque 2.74 2.562.79 (dNm) T90 16.7 7.57 6.78 Stress Strain (cured 40 minutes at 150° C.Tensile Strength 19.14 17.84 20.34 (MPa) Elongation at break (%) 473.9378.6 427.2 300% Modulus (MPa) 10.77 13.41 12.97 RPA (150° C. curecycle, 11 Hz, 100° C.) G′, 10% strain 1734 1570 1669 Tan δ, 10% strain0.101 0.090 0.098 Zwick Rebound RT 34.98 35.21 34.98 100° C. 67.21 73.4170.7 Peal Tear 61.49 47.46 23.53 Tear Appearance Knotty Knotty Smooth

While certain representative embodiments and details have been shown forthe purpose of illustrating the subject invention, it will be apparentto those skilled in this art that various changes and modifications canbe made therein without departing from the scope of the subjectinvention.

1. A tire which is comprised of a generally toroidal-shaped carcass withan outer circumferential tread, two spaced beads, at least one plyextending from bead to bead and sidewalls extending radially from andconnecting said tread to said beads, wherein said tread is adapted to beground-contacting, and wherein said tread is comprised of (1) a rubberypolymer, (2) a silica coupling agent, and (3) a surface metalatedsiliceous filler, wherein the surface metalated siliceous filler ismetalated with a mixture of catalytically active metals selected fromthe group consisting of titanium, zirconium, niobium, tantalum, hafnium,nickel, copper, tin, zinc, cobalt, antimony, manganese, chromium,vanadium, molybdenum, and iron, wherein the titanium is present in themixture at a level of at least 80 mole percent.
 2. The tire as specifiedin claim 1 wherein said composition is essentially void of titaniumdioxide.
 3. The tire as specified in claim 1 wherein the surfacemetalated siliceous filler includes a moiety that contains thecatalytically active metal, wherein the moiety containing thecatalytically active metal is covalently bonded to the surface of thesiliceous filler.
 4. The tire as specified in claim 1 wherein thesurface metalated siliceous filler includes a moiety that contains thecatalytically active metal, wherein the moiety containing thecatalytically active metal is hydrogen bonded to the surface of thesiliceous filler.
 5. The tire as specified in claim 1 wherein thesurface metalated siliceous filler includes a moiety that contains thecatalytically active metal, wherein the moiety containing thecatalytically active metal is bonded to the surface of the siliceousfiller through coordination of the metal to an oxygen atom.
 6. The tireas specified in claim 1 wherein the titanium is present in the mixtureat a level of at least 90 mole percent.
 7. The tire as specified inclaim 1 wherein the titanium is present in the mixture at a level of atleast 95 mole percent.
 8. A tire which is comprised of a generallytoroidal-shaped carcass with an outer circumferential tread, two spacedbeads, at least one ply extending from bead to bead and sidewallsextending radially from and connecting said tread to said beads, whereinsaid tread is adapted to be ground-contacting, and wherein said tread iscomprised of (1) a rubbery polymer, (2) a silica coupling agent, and (3)a surface metalated siliceous filler, wherein the surface metalatedsiliceous filler is metalated with a catalytically active metal selectedfrom the group consisting of titanium, zirconium, niobium, tantalum,hafnium, nickel, copper, tin, zinc, cobalt, antimony, manganese,chromium, vanadium, molybdenum, and iron, and wherein at least 10 molepercent of the catalytically active metal present in said tread ischemisorbed onto the surface of the siliceous filler.
 9. The tire asspecified in claim 8 wherein at least 25 percent of the catalyticallyactive metal present in the tread is chemisorbed onto the surface of themetalated siliceous filler.
 10. The tire as specified in claim 8 whereinat least 50 percent of the catalytically active metal present in thetread is chemisorbed onto the surface of the metalated siliceous filler.11. The tire as specified in claim 8 wherein the catalytically activemetal is titanium.
 12. The tire as specified in claim 11 wherein atleast 25 percent of the catalytically active titanium present in thetread is chemisorbed onto the surface of the metalated siliceous filler.13. The tire as specified in claim 11 wherein at least 50 percent of thecatalytically active titanium present in the tread is chemisorbed ontothe surface of the metalated siliceous filler.
 14. A method of producingan elastomeric composition that is reinforced with a surface metalatedsiliceous filler, said method comprising the steps of: (1) chemisorptinga metal compound onto the surface of silica to produce a surfacemetalated siliceous filler, wherein the metal compound is selected fromthe group consisting of titanium, zirconium, niobium, tantalum, hafnium,nickel, copper, tin, zinc, cobalt, antimony, manganese, chromium,vanadium, molybdenum, and iron compounds, and (2) blending the surfacemetalated siliceous filler and a silica coupling agent into a rubberypolymer.
 15. The method as specified in claim 14 wherein the metalcompound is a titanium compound.
 16. The method as specified in claim 15wherein the titanium compound is tetraisopropyl titanium.
 17. The methodas specified in claim 15 wherein the silica is dried prior tochemisorpting the metal compound onto its surface.
 18. The method asspecified in claim 17 wherein a solution of the metal compound issprayed onto the surface of the silica to facilitate the chemisorptionof the metal compound onto the surface of the silica.
 19. The method asspecified in claim 18 wherein the solution of the metal compound is asolution of tetraisopropyl titanium in isopropanol.
 20. The method asspecified in claim 19 wherein the silica is dried in a vacuum oven at anelevated temperature.